Odontoidectomy and Atlantoaxial Arthrodesis for Treatment of a Malformed Dens in an African Lion (Panthera leo)
An 8-month-old 54-kg castrated male African lion (Panthera leo) was referred for diagnosis and treatment of chronic progressive ataxia. On presentation, the lion had difficulty rising from lateral recumbency and exhibited signs of discomfort when his neck and limbs were manipulated. Incoordination, ataxia, and quadriparesis were observed during the brief periods of ambulation. Physical examination under anesthesia revealed an incipient cataract OD, rear limb muscle atrophy, and mild deviation of the neck to the right that could not be manually straightened. Needle electromyography revealed fibrillation potentials in the platysma muscle adjacent to C1-C2. Survey radiographs of both shoulders and the entire spine did not reveal abnormalities. Myelography by cisternal injection revealed an extradural compressive cervical spinal cord lesion due to dorsal placement of the odontoid process (dens) of the axis. Dorsal and ventral subarachnoid spaces were attenuated at this site. During flexion of the head, the width of the spinal cord was approximately half of the width of the adjacent spinal cord at C2. Following recovery from anesthesia, it was evident that the cervical manipulations had exacerbated the clinical signs. The lion demonstrated quadriplegia with inability to rise and ambulate. Acetylcysteine (Roxane Laboratories, Columbus, OH, USA; 1 g PO every 8 hours × 5 days followed by 1 g PO every 8 hours every other day for 6 days) was administered until surgery could be performed 72 hours after presentation. With the lion in dorsal recumbency, a midline ventral approach was made to reach the cranial cervical vertebrae. The atlantoaxial articulation was identified and exposed. The dorsally displaced dens was removed using a power bur and a bone rongeur to allow decompression of the spinal cord. The cartilage of the articular facets of the atlas and the axis was removed using a power bur. Subluxation and right flexion of the atlantoaxial articulation were reduced. Cancellous bone screws (6.5 mm) were placed through the facets of the axis and atlas. Cancellous bone was collected from the proximal humerus, mixed with a synthetic bone graft particulate (Consil™, Bioglass®, USBiomaterials Corporation, Nutramax Laboratories, Inc., Baltimore, MD, USA), and packed into the atlantoaxial joint space to encourage fusion of C1-C2. Postoperative radiographs confirmed surgical removal of the dens, proper screw and graft placement, and adequate alignment of the vertebrae. Within 24 hours following surgery, the lion made attempts to stand, began to regain strength in all limbs, and exhibited fewer signs of discomfort during manipulation and movement. Two weeks following odontoidectomy and atlantoaxial arthrodesis, the lion moved easily and playfully with only mild to moderate incoordination.
Treatment centered on surgical decompression of the spinal cord, surgical stabilization to prevent further damage to the spinal cord, and medical therapy with acetylcysteine to reduce free radical and oxidative damage to spinal cord tissue.2 The thickness of the wings of the atlas in this animal allowed placement of 6.5 mm cancellous bone screws. Unique to this procedure was the utilization of a bioactive ceramic (Bioglass®) in conjunction with bone graft to aid regeneration of bone.5 This synthetic particulate product initiates a rapid chemical bond to bone3,4 and shows osteoproductive properties. After exposure to blood in the surgical wound, the surface of Bioglass® converts to a silica-rich gel layer that mineralizes into a hydroxyapatite layer. Osteogenic stem cells in the bone graft-Bioglass® admixture colonize the bioactive surface of the synthetic particulate.6 Osteoblasts then differentiate and produce new bone throughout the implant site.1
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